(1) Field of the Invention
This invention relates to a process for producing a carbon fabric having improved tenacity, elongation and modulus of elasticity.
(2) Description of the Prior Art
In general, a carbon fabric is manufactured by a process wherein a precursor fiber is heated at a temperature of from 200.degree. to 400.degree. C. in an oxidizing gas atmosphere thereby to be stabilized; then, the heat-treated fiber is carbonized at a temperature of at least 1,000.degree. C. in a non-oxidizing gas atmosphere. Particularly, in order to produce a carbon fiber of improved tenacity and modulus of elasticity, it has been proposed that an acrylonitrile polymer fiber be employed as the precursor fiber and that the heat treatment of the acrylonitrile polymer fiber in an oxidizing gas atmosphere be carried out while tension is being applied to the acrylonitrile polymer fiber. However, this proposed technique involves the following problems. Mechanical properties of the final carbon fibers are liable to be not uniform due to possible slight variations in the tension applied to the precursor fibers during the heat treatment step. Occasionally, fibers have become fuzzy or broken during the heat treatment. Therefore, it would be necessary to strictly control the tension applied to the fibers and to use precursor fibers having uniform internal structure and physical properties.
Furthermore, when a fabric is used as the precursor instead of a fiber such as a filament or tow, the following problem occurs. It is difficult to apply a desired degree of uniform tension to fabrics during heat treatment; therefore, carbon fabrics thus treated will exhibit poor mechanical properties such as poor tenacity and modulus of elasticity.
U.S. Pat. No. 3,497,318 discloses a process for preparing carbon textiles wherein a thermoplastic polyacrylonitrile base textile material is subjected to a chemical oxidizing agent such as potassium permanganate, thereby to be converted into a thermoset polyacrylonitrile base textile material; and then, the thermoset polyacrylonitrile base textile material is carbonized. This reference specifically discloses the use of a yarn or fiber as the thermoplastic polyacrylonitrile base textile material, but does not specifically disclose a fabric. In this process, the oxidizing treatment by the chemical oxidizing agent is carried out with no tension being applied to the yarn or fiber, and hence, the finally resulting carbon fiber has poor mechanical properties. This is true even if a fabric is used instead of the yarn or fiber as the thermoplastic polyacrylonitrile base textile material.
U.S. Pat. No. 3,285,696 discloses a process wherein precursor acrylonitrile polymer fibers or fabrics are heated at a temperature of from 180.degree. to 550.degree. C. in an oxygen-containing atmosphere for a time sufficient to blacken the fibers or fabrics, and then, the blackened fibers or fabrics are carbonized. However, no special consideration is given to the precursor fibers or fabrics and, the heating conditions, other than the heating temperature and time. It is difficult to prepare carbon fabrics having satisfactory mechanical properties by this process.
U.S. Pat. No. 3,945,093 discloses a process wherein precursor polyacrylonitrile fabrics are heated under warp tension while restraining fill shrinkage by using a multiple roller unit. It is difficult, however, to apply uniform tension to the precursor fabrics, and therefore, the resulting carbon fabrics are undesirably hard and exhibit poor mechanical properties.
French Pat. No. 2,097,981 teaches that the heat oxidation treatment of precursor acrylonitrile copolymer fibers under tension, e.g. under a tension of 100 mg/denier, result in fibers which can be carbonized into carbon fibers of improved mechanical properties. However, even when precursor acrylonitrile copolymer fabrics are subjected to the heat oxidation treatment according to this teaching, the finally resulting carbon fabrics would be undesirably hard and exhibit poor mechanical properties.
Japanese Patent Publications No. 47-26,974 and Japanese Laid-open Patent Application No. 49-527 disclose the preparation of carbon fibers from precursor polyacrylonitrile fibers, but are completely silent on the preparation of carbon fabrics from precursor polyacrylonitrile fabrics.